1. Field of the Invention
The present invention relates to a diffractive optical element and an optical system having the same, which are suitable for an image pickup optical system, an observation optical system, or the like.
2. Description of the Related Art
Conventionally, a method of reducing a chromatic aberration of a lens system (an optical system) by providing a diffractive optical element having a diffraction grating in a part of the lens system has been known. Generally, the diffraction grating is constituted by a blazed structure constituted of a grating surface and a grating side surface. The diffraction grating having the blazed structure can diffract light with high efficiency for diffracted light having a specific one order (hereinafter, referred to as a “specific order” or a “designed order”) and a specific wavelength. A structure of the diffractive optical element for obtaining sufficiently-high diffraction efficiency of the specific order in an entire visible wavelength range is known (Japanese Patent Laid-open No. H9-127321). The diffractive optical element disclosed in Japanese Patent Laid-open No. H9-127321 constitutes a diffraction grating part by adhesively arranging two diffraction gratings and a material of each diffraction grating and a height of the grating part of each diffraction grating are appropriately set (hereinafter, such a diffraction grating part is referred to as a “contacting two-layer DOE”). With respect to diffracted light having a desired order, high diffraction efficiency is realized over a wide wavelength range. The designed order is not defined. The diffraction efficiency is represented as a ratio of light intensity of the diffracted light of each order with respect to the light intensity of entire transmitted luminous flux.
The luminous flux which enters the grating wall surface of the grating part forming the diffraction grating exhibits behaviors such as reflection and refraction which are different from those on the grating surface, and as a result, it generates unnecessary light (flare). Therefore, a diffractive optical element which suppresses generating the unnecessary light on the grating wall surface is known (Japanese Patent Laid-open No. 2005-292571). The diffractive optical element disclosed in Japanese Patent Laid-open No. 2005-292571 optimizes an angle of the grating wall surface with respect to luminous flux entering the diffractive optical element at an obliquely incident angle (an off-screen light incident angle) with reference to a principal ray incident angle (a designed incident angle or an imaging light incident angle). Thus, it prevents the unnecessary light generated on the grating wall surface from reaching the imaging plane. With respect to the designed order, a diffractive optical element having the designed order equal to or greater than +2nd order is known (Japanese Patent Laid-open No. 2005-70124). Japanese Patent Laid-open No. 2005-70124 discloses a diffractive optical element having the designed order equal to or greater than +2nd order, and the grating size of the grating part is large so as to be easily manufactured.
In an optical system using the diffractive optical element, the unnecessary light is generated with respect to the luminous flux entering the diffractive optical element at an obliquely incident angle (an off-screen light incident angle) with reference to a principal ray incident angle. When a part of the unnecessary light reaches an imaging plane, the image performance is deteriorated. With respect to this problem, previously, behaviors of the luminous flux entering the grating surface and the grating wall surface of the grating part of the diffraction grating are separately considered, and the behavior on the grating surface is treated as a diffraction phenomenon caused by the blazed structure, and the behavior on the grating wall surface is treated as a phenomenon of a geometric optics. However, it is difficult to treat the grating surface and the grating wall surface of the grating part separately, and in the conventional method, the unnecessary light reaching the imaging plane is not sufficiently suppressed.